Being interested in the application of Tchebycheffian splines in IgA, it is important to understand the approximation power of the space in (REF ). It turns out that, in general, smooth functions can be approximated by Tchebycheffian splines with the same orders of approximation as in the case of classical polynomial splines. The reader is referred to [1]} for details and we state a simplified version of [1]} below. Let \(\mathbb {W}^{r,q}([a,b])\)  be the standard Sobolev space on \([a,b]\)  and let \(\Vert \cdot \Vert _q\)  denote the standard \(L^q\) -norm on \([a,b]\)  for \(1\le q\le \infty \) .
